1. Field of the Invention
The present invention relates to a system and method for transmitting and receiving parallel signals between devices interconnected in a computer or between computers at a relatively short distance, via an optical fiber.
2. Description of the Related Art
A well-known conventional parallel signal transmission system with an optical transmission path is Fiber Channel system which has been standardized by American National Standards Institute (hereinafter ANSI) (see ANSI X3T11 FC-PH REV4.3). According to the Fiber Channel system, 8-bit parallel signals inputted to the transmission unit of the system are sent to the reception unit as follows.
FIG. 1 is a block diagram showing the conventional parallel signal transmission system, which is composed of a transmission unit A, a transmission path B, and a reception unit C.
In the transmission unit A, 8-bit parallel signals inputted to the terminals 1-8 are first converted into 10-bit parallel signals in the 8B/10B conversion circuit 10, by adding 2 bits for synchronization. Then, the 10-bit parallel signals are converted into serial signals in the parallel/serial conversion circuit 11 for serial transmission.
Here, for the framing of the reception unit C, the frame sync signal generation circuit 50 outputs frame sync signals to the parallel/serial conversion circuit 11 in a certain time period immediately after the system has been powered on, based on a control signal inputted to the terminal 9. The frame sync signals are either 0011111010 or 1100000101 in the case of the Fiber Channel system.
Then, the parallel/serial conversion circuit 11 makes the drive circuit 12 drive a semi-conductor laser diode (hereinafter LD) 13 having a 0.78 .mu.m wavelength to output intensity-modulated optical signals. Generally, the intensity of lights to be outputted is changed in proportion to the amount of the current to be supplied to LDs.
LDs use a light which is generated from the recombination of carriers by supplying current which amounts over the threshold in the forward direction of P-N connection. Having their own resonance structure, LDs can exhibit a light having high coherency. The oscillation mode of LDs can be either a vertical mode in which an electromagnetic field is formed in the direction for a light to expand, or a horizontal mode in which an electromagnetic field is formed in the direction vertical of the light expansion. Generally, the sizes of waveguides are fixed to allow LDs to oscillate based on the horizontal mode, in view of efficient light coupling with an optical circuit such as an optical fiber, the proportional relation between outputted lights and the current amount, modulation in a wide band width, low noise, and the like.
Out of LDs having a 0.78 .mu.m wavelength and a 1.3 .mu.m wavelength standardized by the ANSI, LDs having a 0.78 .mu.m wavelength are popular for use in compact discs and easily available as a light emitting element. The LD 13 having a 0.78 .mu.m wavelength emits a light in an approximate range of .+-.10 degrees in the horizontal direction and .+-.20 degrees in the vertical direction.
Then, in the transmission path B, optical signals outputted from the LD 13 are converged by the lens 14 and sent through the graded index fiber 15 (hereinafter GI 15) whose core diameter is either 50 or 62.5 .mu.m. Since the GI 15 can receive a light in the range of .+-.11.5 degrees, the lens 14 is used in order to secure the coupling efficiency between the LD 13 and the GI 15.
Generally, optical fibers are superior to other transmission media in flexibility, lightness in weight, signal transmission stability to temperature, or the like. They are composed of a core having a large index of refraction and a clad having a smaller index of refraction which coats the core. Optical signals are sent through the core, repeating a total reflection on the border surface between the core and the clad.
Optical fibers are classified into single mode type (hereinafter SM type) fibers and multi mode type (hereinafter MM type) fibers, depending on their optical signal propagation modes. The SM type fibers have a small core diameter of about 10 .mu.m so that only one propagation mode exists, and as a result, the wide transmission band width allows signals to be transmitted at a high speed of 500 Mbps or higher. However, the SM type fibers are hard to be connected because of their small core, so that they do not lead to cost reduction.
On the other hand, the MM type fibers are further classified into step index (hereinafter SI) type fibers and graded index (hereinafter GI) type fibers.
The SI type fibers have an about 1 mm core diameter, and their index of refraction changes in the form of stairs on the border of the core and the clad. This type of fibers are mainly used in the field of controlling the transmission of signals of 50 Mbps or smaller, for example factory automation.
The GI type fibers have a core diameter of about 50 .mu.m, and their index of refraction is not uniform but gradually diminished toward the outermost surface of the core. This design makes optical signals be propagated windingly or spirally against the axis of the optical fibers. Thus, the MM type fibers can provide various propagation modes because of their large core diameter, and as a result, the transmission band width is relatively smaller than that of SM type fibers. Consequently, the GI type fibers having a wider transmission band are more commonly used than the SI type fibers, for the purpose of transferring signals of around 200-500 Mbps between devices in a computer or between computes at a relatively short distance.
Then, in the reception unit C, optical signals sent through the GI 15 are photo/electric converted with a light reception element 16, and further amplified in the amplification circuit 17 to obtain serial signals having a fixed amplitude. The serial signals are converted into 10 bit parallel signals in the serial/parallel conversion circuit 18.
In the frame sync signal detection circuit 51, when a frame sync signal having a predetermined pattern has been detected, it is regarded that a frame synchronization has been established, then every 10-bit serial signals which follow the detected frame sync signal are converted into parallel signals. Then, the parallel signals sent from the serial/parallel conversion circuit 18 are converted back into 8-bit parallel signals in the 10B/8B conversion circuit 19 and outputted through the terminals 21-28. A light reception element is selected depending on the wavelength of an optical signal coming into the element. Generally, a silicon pin photo diode is used together with the LD 13 having a 0.78 .mu.m wavelength. The core diameter of the light reception element can be from 80 .mu.m and 1 mm; however, the larger the core diameter is, the smaller the frequency band width of the light reception unit is. This makes high speed signal transmission difficult. This is the reason that a light reception element having a core diameter of around 100 .mu.m is generally used with a GI type fiber having a core diameter of 50 .mu.m. When an LD having a 0.78 .mu.m wavelength is used, the angle of a light going out from the GI type fiber is around 1.1 degrees.
As described hereinbefore, according to the conventional parallel signal transmission system, 8B/10B codes are used as transmission signals to drive the LD 13 to output intensity-modulated optical signals. As a result, some 10-bit parallel signals may lengthen the duration of light emission of the LD 13, causing mode hopping noises, which are inherent to the LD 13.
To be more specific, the oscillating wavelength of LDs tends to be longer in accordance with the increase in the index of refraction when the temperature of the LDs or the amount of current to be supplied thereto are raised. This causes the vertical mode to hop, and as a result, oscillating wavelength hopping is generated, which is accompanied with noises. These noises are called mode hopping noises, which appear as irregular intensity of a laser beam.
In the case of 8B/10B codes, "1" values may be consecutive for at most 6 bits in accordance with the coding rule. Consequently, for example, in the case of signals having a bit rate of 192 Mbps, the transmission rate turns out to be 192.times.(10/8)=240 on the transmission path, and as a result, the duration of light emission of the LD 13 becomes (1/240M).times.6 =25 ns. In the same manner, in the cases of signals having 300 Mbps, 400 Mbps, and 500 Mbps, the duration of the light emission of the LD 13 becomes 16 ns, 12 ns, and 9.6 ns, respectively. Thus, the longer the duration of the LD 13 is, the higher the possibility of the occurrence of the mode hopping noises is. The mode hopping noises are the first factor of the generation of burst bit errors in received signals.
When either 0 values or 1 values are consecutive, low frequency components increase, causing the fluctuation of DC components in the transmission path signals. The fluctuation of the DC components changes, for example, the operational bias potential of the amplification circuit 17 in the reception unit C, so that it gets harder to correctly demodulate signals. To restrict the fluctuation of the DC components, the reception unit C processes signals based on a DC voltage of its own regulation, without relying on received signals. This process is generally called DC reproduction, which is achieved by integrating a DC reproduction circuit composed of a clamping circuit into the amplification circuit 17.
FIG. 2 shows the construction of a specific DC reproduction circuit, and the waveforms of pulse signals to be inputted thereto and outputted therefrom, in order to explain the operation of the DC reproduction circuit. As apparent from FIG. 2, the low level of an inputted pulse signal is supposed to be clamped by the anode voltage of the LD 13. Thus, pulse signals to be outputted are supposed to be operated based on the anode voltage, regardless of the signs of the pulse signals.
However, the DC reproduction circuit is susceptible to the effects of low frequency noise, which mainly results from resistance, heat noise in diode and useless signals which come from other circuits. Because of the effects of the low frequency noise, the low level (or high level) of the pulse signals to be outputted from the DC reproduction circuit is not fixed but undesirably sways in the case of a low frequency. Consequently, the use of a DC reproduction circuit becomes the second factor of the occurrence of burst bit errors when the sign is identified in the next stage.
Since the GI 15 has a relatively small core diameter of from 50 to 62.5 .mu.m, a small error in the size of the connecting portion of a connecter or a splice may have great effects, causing modal noises at high probability. For example, a 5 .mu.m divergence of the light axis for a core diameter of 50 .mu.m would affect about 10% of the core diameter.
The modal noises occur when a laser beam having excellent coherency is transmitted by means of a multi mode fiber, as a result of the difference in the optical transmission path lengths between the low mode and the high mode. To be more specific, the difference in light path lengths causes interference fringe, which changes as time goes by the vibration of the fiber or heat disturbance. If the entire amount of light is received, the light path lengths are averaged, causing no noise. However, if a part of the light is received because of the divergence of the light axis at the connecting portion of the fiber or the like, modal noises are generated. The modal noises become the third factor of the occurrence of burst bit errors.
As described hereinbefore, any of the mode hopping noises generated in LDs, low frequency noise generated in the reception unit, and the modal noises generated in an optical fiber may cause burst bit errors when signals are demodulated in the reception unit. It is hardly possible to avoid the occurrence of these noises by the conventional parallel signal transmission system. In order to secure a high quality signal transmission performance for a long period of time, it is necessary to provide a signal process circuit for performing a complicated process such as error correction, and also to use a precision light connector. However, this inevitably raises the cost.